1. Field of the Invention
The present invention relates generally to process control networks and more particularly to tools for analyzing process control network designs.
2. Description of the Related Art
Large processes such as chemical, petroleum and other manufacturing and refining processes include numerous field devices disposed at various locations within a facility to measure and control process parameters which thereby effect control of the process. These devices may be, for example, sensors such as temperature, pressure and flow rate sensors as well as control elements such as valves and switches. Historically, the process control industry used manual operations such as manually reading level and pressure gauges, turning valve wheels, etc., to operate the measurement and control field devices within a process. Beginning in the 20th century, the process control industry began using local pneumatic control, in which local pneumatic controllers, transmitters and valve positioners were placed at various locations within a process plant to effect control of certain plant locations. With the emergence of the microprocessor-based distributed control system of the 1970""s, distributed electronic process control became prevalent in the process control industry.
A distributed control system includes an analog or digital computer, such as a programmable logic controller, connected to numerous electronic monitoring and control devices, such as electronic sensors, transmitters, current-to-pressure transducers, valve positioners, etc. located throughout a process. The DCS computer stores and implements a centralized and, frequently, complex control scheme to effect measurement and control of process parameters according to an overall control scheme. Usually, however, the control scheme implemented by a DCS is proprietary to the DCS manufacturer. Thus making the DCS difficult and expensive to expand, upgrade, reprogram and/or service because the DCS provided must become involved in an integral way to perform any of these activities. Furthermore, the equipment that can be used by or connected to any particular DCS may be limited due to the proprietary nature of the DCS and that a DCS provided may not support certain devices or functions of devices manufactured by other vendors.
To overcome some of the problems inherent in the use of proprietary DCSs, the process control industry has developed a number of standard, open communication protocols including, for example, the HARTS(copyright), DE, PROFIBUS(copyright), WORLDFIP(copyright), LONWORKS(copyright), Device-Net(copyright), and CAN protocols. These standard protocols enable field devices made by different manufactures to be used together within the same process control environment. In theory, any field device that conforms to one of these protocols can be used within a process to communicate with and to be controlled by a DCS or other controller that supports the protocol, even if that field device is made by a different manufacturer that the DCS manufacturer.
There is now a move within the process control industry to decentralize process control and, thereby, simplify DCS controllers, or to a large extent, eliminate the need for DCS controllers. Decentralized control is provided by having process control devices, such as valve positioners, transmitters, etc. perform one or more process control functions and by then communicating data across a bus structure for use by other process control devices. To implement control functions, each process control device includes a microprocessor having the capability to perform one or more basic control functions as well as the ability to communicate with other process control devices using a standard and open protocol. In this manner, field devices made by different manufacturers can be interconnected within a process control loop to communicate with one another and to perform one or more process control functions or control loops without the intervention of a DCS. One example of an open communication protocol that allows devices made by different manufacturers to interoperate and communicate with one another via a standard bus to effect decentralized control within a process is the FOUNDATION Fieldbus protocol (hereinafter the xe2x80x9cFieldbus protocolxe2x80x9d) by the Fieldbus Foundation. The Fieldbus protocol is an all digital, two-wire loop protocol.
When using these protocols, a challenge associated with designing the process control system or network relates to the actual physical layout and interconnection of the various process control devices. Specifically, each of these protocols sets forth constraints of values for the physical characteristics within which a process control system must operate to conform to the standard. These constraints include the voltage drop across communication sections, the spur length, the overall cable length, the total current draw and the total number of process control devices on a particular hub. The interrelationship of these constraints are important and variable based upon the values of the constraints.
Presently, when designing a process control system, the physical layout of the system is designed and physically drawn and then the values of the physical characteristics of the system are calculated and recalculated by hand to determine whether the values are within the constraints set forth by a particular protocol. This process can be time consuming as the values of the physical characteristics are interrelated and changing. One change in value may require the recalculation of the values of the physical characteristics of the entire process control environment.
The present invention is directed to an analysis tool for aiding in the design of a process control system which conforms to a standard protocol. Such a tool advantageously allows the efficient design of a process control system while ensuring that the physical characteristics of the system conform to the standard.
One aspect of the invention is directed to a method for analyzing a process control network design to meet criteria of a standard protocol. The process control network includes a controller coupled to a field device via a bus. The method includes a software analysis tool having access to information regarding standard protocol criteria including a length of the bus, a cable type of the bus and a voltage requirement of the field device for analysis by the tool to assure that the process control network design conforms to the criteria of the standard protocol.